The present invention relates to pre-expanded particles of non-crosslinked linear low density polyethylene and a process for producing cellular foamed materials therefrom. More particularly, the invention relates to non-crosslinked linear low density polyethylene pre-expanded particles which are applicable, without further imparting expandability to the pre-expanded particles prior to molding, to a process for producing foams by heating pre-expanded particles with a heating medium such as steam in a mold which is able to be closed but unable to be sealed, in other words, in a mold which is able to be closed but is permeable to a fluid heating medium.
In a general process for producing foams by molding polyolefin resins in a mold, particles of the resins containing volatile blowing agents are pre-expanded by heating with steam or the like prior to molding. However, in case of polyethylene resins, it is very difficult to obtain pre-expanded particles which have a high expansion ratio and are less shrinked, because the melt viscosity of polyethylene resins remarkably drops at a temperature near the melting point thereof. Therefore, it is essential to crosslink the polyethylene resins. High pressure process low density polyethylene has been generally used as a starting material for preparing foams of crosslinked polyethylene resins because of good crosslinkability. The foamed articles of the crosslinked high pressure process low density polyethylene are superior in flexibility and buffer or cushioning action. However, for the reason that they are inferior in heat resistance and come short of rigidity, the pre-expanded particles of the crosslinked high pressure process polyethylene have the disadvantage that they must be used in a relatively low expansion ratio.
In order to eliminate the above-mentioned disadvantages, it is proposed to use a linear low density polyethylene (LLDPE) as a polymer material instead of the high pressure process low density polyethylene, as disclosed in Japanese Unexamined Patent Publication (Tokkyo Kokai) No. 50-187036 and Japanese Examined Patent Publication (Tokkyo Kokoku) No. 60-10047. According to these proposed processes, pre-expanded particles of non-crosslinked polyethylene are obtained and they can be used for molding in a mold. However, for obtaining foamed articles from such pre-expanded particles of non-crosslinked linear low density polyethylene, it is necessary, prior to molding the pre-expanded particles, to further impart the expandability to the pre-expanded particles (Japanese Unexamined Patent Publication No. 59-187036) or to conduct aging under pressure (Japanese Examined Patent Publication No. 60-10047).
The present inventors have found that such disadvantages of the known pre-expanded particles of non-crosslinked linear low density polyethylene result from the fact that the average diameter of cells in the pre-expanded particles is as small as about 90 .mu.m or less. For example, the average cell diameter of the pre-expanded particles disclosed in Examples of Japanese Examined Patent Publication No. 60-10047, calculated from the number of cells, is less than 90 .mu.m.
On the other hand, there is known, for example, as disclosed in DE-OS 2,107,683 or Japanese Examined Patent Publication No. 56-1344, a process for pre-expanding particles of polyolefin resins by dispersing the polyolefin resin particles and a volatile blowing agent into water in a pressure vessel such as autoclave, maintaining the dispersion at high temperature and high pressure and allowing the dispersion to release into a low pressure zone. Compared to a conventional process for pre-expansion using heated steam, this process can control the temperature of the resins upon expansion within a narrow range and, therefore, it enables pre-expansion of non-crosslinked polyethylene resins whose temperature range showing a viscoelasticity optimum to expansion is narrow. However, the mechanism of cell formation in the pre-expanded particles obtained by this process is different from that in the process for pre-expansion using heated steam, and it is difficult to control the cell diameter to a desired value. Japanese Unexamined Patent Publication No. 58-199125 discloses decreasing the diameter of cells in pre-expanded particles by using an inorganic fine powder such as talc or silica, but no technique for increasing the diameter of cells has been known.
The average diameter of cells of the pre-expanded particles of non-crosslinked linear low density polyethylene prepared according to the above process, namely by dispersing the non-crosslinked polymer particles and a volatile blowing agent into water in a pressure vessel followed by release of the particles present under high temperature and high pressure into a low pressure zone, varies depending on the manner of preparing the non-crosslinked linear low density polyethylene, the temperature and pressure at the time when the polymer particles are released from a pressure vessel into a low pressure zone, or the like, but is always as small as less than about 90 .mu.m. The pre-expanded particles of non-crosslinked linear low density polyethylene having cells of such a small diameter have the problem that they cannot substantially be molded because the allowable range of heating condition for molding is very narrow. Even if possible, they provide only moldings extremely shrinked.
It is an object of the present invention to eliminate the drawbacks of the pre-expanded particles of non-crosslinked linear low density polyethylene.
A further object of the present invention is to provide pre-expanded particles of non-crosslinked linear low density polyethylene which can be molded, without requiring special procedures, by heating in a closed mold that is permeable to fluids.
A still further object of the present invention is to provide pre-expanded particles of non-crosslinked linear low density polyethylene which have a high expansion ratio and a good moldability and provide foams having a high expansion ratio and good heat resistance, mechanical property, flexibility and cushioning property with less shrinkage.
These and other objects of the present invention will become apparent from the description hereinafter.